A survey of the urea transport properties of nephron segments in the kidney of the rat is being carried out in order to formulate an integrated model of the role of urea in the urinary concentrating mechanism. In recent experiments, net urea fluxes were measured in isolated, perfused cortical and medullary thick ascending limbs and cortical collecting ducts. No evidence for active transport of urea was found. The urea permeability was found to be relatively low in thick ascending limbs from the inner stripe of the outer medulla, but substantially higher in thick ascending limbs from the outer stripe and medullary rays. The relatively high urea permeability of thick ascending limbs in the outer stripe of the outer medulla and medullary rays is likely to permit substantial passive absorption of urea from these segments in vivo. In cortical collecting ducts, the urea permeability was relatively low both in the presence of 100 MuU/ml arginine vasopressin in the bath in its absence.